MCJ is a recently identified member of the DnaJ protein family of co-chaperones and its expression is controlled by methylation (Shridhar et al., Cancer Res 61, 4258-4265, 2001). DnaJ polypeptides are characterized by the presence of the DnaJ domain containing the His-Pro-Asp signature tripeptide. The DnaJ protein family is one of the largest co-chaperone families that has members with diverse cellular localization and functions (reviewed by Craig et al., Rev Physiol Biochem Pharmacol 156, 1-21, 2006). In addition to the DnaJ family, two other families of co-chaperones have been identified based on the presence of the Bag domain (Sondermann et al., Science 291, 1553-1557, 2001) or the tetratricopeptide repeat (TRP) clamp domain (Scheufler et al., Cell 101, 199-210, 2000; Sonderinann et al., Science 291, 1553-1557, 2001)). Co-chaperones associate with the heat shock protein (Hsp) 70 (Hsp90, Hsp70, Hsc70) family of chaperones through these conserved domains and participate in protein folding and trafficking (reviewed by Young et al., Trends Biochem Sci 28, 541-547, 2003). Co-chaperones have a modular architecture in which a chaperone-binding domain (DnaJ, TRP or Bag) is fused to other non-conserved sequences that can interact with specific proteins and mediate a variety of diverse activities including clathrin uncoating (Ungewickell et al., Nature 378, 632-635, 1995) and cytoskeletal function (Izawa et al., J Biol Chem 275, 34521-34527, 2000). Some DnaJ co-chaperones also participate in ubiquitin dependent proteolysis either by tagging certain substrates for degradation or by facilitating the unfolding of folded proteins thus allowing degradation by proteolysis (Lee et al., Mol Cell Biol 16, 4773-4781, 1996).
MCJ has some unique features among the members of the DnaJ family. It is a rather small polypeptide of 150 aa (16-17 kDa) as compared to other members (˜40 kDa). The DnaJ domain is located in the C-terminus (Shridhar et al., Cancer Res 61, 4258-4265, 2001), while it is commonly present in the N-terminus in other DnaJ proteins. In addition, a potential transmembrane domain distinguishes MCJ from most other DnaJ proteins that are present in the cytosol and interact with chaperones through the DnaJ domain. Thus, MCJ appears to be an atypical DnaJ family member.
MCJ was identified as a gene expressed in normal ovarian epithelial cells, but absent or expressed at very low levels in a number of primary ovarian tumors and ovarian carcinoma cell lines (Shridhar et al., Cancer Res 61, 4258-4265, 2001). Loss of MCJ was correlated with increased drug resistance in ovarian cancer cell lines (Shridhar et al., Cancer Res 61, 4258-4265, 2001). Hypermethylation of a CpG island present within the first exon and first intron of the MCJ gene represses MCJ expression (Strathdee et al., Carcinogenesis 25, 693-701, 2004). Overexpression of MCJ in ovarian cancer cells increases sensitivity to anti-neoplastic drugs in vitro (Shridhar et al., Cancer Res 61, 4258-4265, 2001). A recent study in ovarian cancer patients demonstrates that the high levels of CpG island methylation correlates with poor response of these tumors to chemotherapy and overall poor survival (Strathdee et al., Gynecologic Oncology 97, 898-903, 2005). Methylation of the MCJ gene has also been reported in some malignant pediatric brain tumors and in 90% of Wilms tumors, whereas very low levels of methylation has been found in normal tissues (Ehrlich et al., Oncogene 21, 6694-6702, 2002; Lindsey et al., Int J Cancer. 2006 Jan. 15; 118(2):346-52, 2005). However, the relevance of MCJ gene hypermethylation for chemoresistance in these tumors has not yet been addressed.
Although regulation of MCJ gene expression has received certain amount of interest, no information about the biology and function of the MCJ protein, including its cellular localization, is currently available. In addition, although the loss of MCJ gene expression by hypermethylation has been correlated with multidrug resistance in ovarian cancer, the mechanism by which this co-chaperone regulates the drug response is completely unknown.